The invention concerns a spark ignition internal combustion engine with at least one ignition device per cylinder wherein at least one piston has a recess in the piston head.
Internal combustion engines with pistons of that kind are already known, wherein the recess can in part be of a bowl or tub shape (Hyronen combustion chamber). It can be disposed centrally or eccentrically in the piston. In addition, EP 0 598 941 A1 and EP 0 299 747 A2 also disclose piston recesses which are of a configuration that differs from the simple bowl or tub shape. The purpose of such a recess is to improve engine combustion and in particular to achieve very fast, stable and anti-knock combustion. That can only be attained by ensuring optimum turbulence or flow speed in the region of the ignition location of the spark plug. In that respect it is to be noted that, with excessively high flow speeds in the region of the ignition location the ignition spark is blown out while with excessive low flow speeds in the region of the ignition location combustion takes place too slowly and too sluggishly. In the internal combustion engines which are already known, with recesses in the piston head, in accordance with the state of the art, an optimum flow speed in the region of the ignition location is only inadequately ensured.
Therefore the object of the present invention is to design a recess in the piston head in such a way that the above-mentioned disadvantages of the prior art are eliminated and optimum and fast combustion in the combustion chamber of spark-ignition internal combustion engines is ensured.
In accordance with the invention that is achieved in that the recess in plan view on to the piston head is of a substantially heart-shaped shape which differs at least in a region-wise manner from a circular shape, wherein the shape of the recess in a plan view on to the piston head has precisely one axis of symmetry.
The configuration of the recess in the piston head, in accordance with the invention, provides that the flow speed at the ignition location is in the optimum range (between about 4 m/sec and 6 m/sec). That therefore ensures that the ignition spark at the ignition location is neither blown out by excessively high flow speeds nor does excessively slow combustion of the fuel-air mixture take place due to excessively low flow speeds at the ignition location. Ensuring an optimum flow speed at the ignition location, in accordance with the invention, provides for very fast combustion of the fuel-air mixture in the entire combustion chamber, as is otherwise known only in relation to compression-ignition internal combustion engines (for example diesel engines). The invention can preferably be used in Otto-cycle gas engines.
In that respect a particularly advantageous embodiment provides that the recess is of a non-convex shape in a plan view on to the piston head. In this respect the term xe2x80x98non-convexxe2x80x99 relates to the fact that, in a plan view on to the piston head, not all connecting lines between any two points on the periphery of the shape of the recess according to the invention extend within the recess.
Advantageous variants of the recess according to the invention are in this connection characterised in that in a plan view on to the piston head the recess, over an angular region of between 200xc2x0 and 250xc2x0, preferably between 220xc2x0 and 230xc2x0, is of a substantially circular peripheral shape, preferably with the center point of the circular piston area as the center of the circle.
Further advantageous embodiments provide that in a plan view on to the piston head the recess has an inwardly extending portion. It is desirable in that respect if in a plan view on to the piston head the inwardly extending portion extends over an angular region of between 80xc2x0 and 130xc2x0, preferably between 100xc2x0 and 110xc2x0. In addition it is desirable if in a plan view on to the piston head the inwardly extending portion extends into the interior of the recess over a maximum extent of between 25% and 50%, preferably between 35% and 40%, of the largest diameter of the recess.
It is further important for the ignition location of the ignition device to be arranged at a favourable position with respect to the recess. In that respect a highly favourable alternative configuration provides that at least one ignition device has an ignition location, wherein the ignition location in projection on to the plan view on to the piston head is substantially on the axis of symmetry of the recess and/or substantially at the center point of the circle of the circular region of the peripheral shape of the recess.
With regard to the arrangement of the recess according to the invention in relation to the inlet and exhaust valves of the cylinder of the internal combustion engine, it is desirable if in a plan view on to the piston head the axes of symmetry of the arrangement of the inlet and exhaust valves assume an acute angle, which differs from 0xc2x0, 45xc2x0 and 90xc2x0, with respect to the axis of symmetry of the recess. That arrangement provides that the structure involves an asymmetrical position of the recess in accordance with the invention, in relation to the preferred flow directions of the fuel-air mixture flowing in and out through the inlet and exhaust valves. That is in turn particularly desirable in terms of achieving an optimum flow speed for the fuel-air mixture and optimum turbulence of the fuel-air mixture at the ignition location.
In order to achieve adequate cooling of the piston with the recess according to the invention, it is particularly desirable for a cooling duct to be arranged in the piston in the material surrounding the recess.
Desirable embodiments further provide that the shape of the recess, as viewed in a plan view on to the piston head, extends substantially over the full depth of the recess.